Serial port is a very common device communication protocol on industrial motherboards, and most industrial motherboards contain more than two serial ports based on RS-232. The serial port is a bit of sending and receiving bytes, which is slower than byte-by-byte parallel communication, but the serial port can use one line to send data and receive data on another line, which can simply complete long-distance communication. The most important parameters of serial communication are baud rate, data bits, abort bits, and parity.
Baud rate: is a measure of communication speed, which indicates the number of bits transmitted per second. For example, 300 baud indicates that 300 bits are sent per second. When referring to clock cycles, we are referring to the baud rate, and if the protocol requires a baud rate of 4800, then the clock is 4800Hz. However, the baud rate is inversely proportional to the interval, and the high baud rate is often used for communication between instruments with short intervals, typically for communication with GPIB devices.
Data bits: are the parameters that measure the bits of practice data in a communication. When an IPC sends a packet, the practice data will not be 8 bits, the standard values are 5, 7 and 8 bits. How you set it up depends on the message you want to send. For example, the ASCII code of the specification is 0-127 (7 bits). The extended ASCII code is 0-255 (8 bits). If the data is in simple text (ASCII code), then each packet uses 7 bits of data. Each packet refers to a byte and includes the start/abort bits, data bits, and parity bits. Since the practice data bits depend on the choice of communication protocol, the term "packet" refers to the situation of any communication.
Abort bit: Used to indicate the last bit of a single packet. Typical values are 1, 1.5, and 2 bits. Since the data is time-bound on the transmission line, and each device has its own clock, it is likely that there will be a small dissynchronization between the two devices in the communication. Therefore, the abort bit not only indicates the end of the transmission, but also provides the timing for the IPC to correct the clock synchronization. The more bits that can be applied to the abort bits, the more tolerant it is to not synchronize with the clock, but the slower the data transfer rate will be.
Parity bit: A simple error detection method in serial communication on industrial motherboards. There are four methods of error detection: even, odd, high, and low. Of course, it is possible without a check digit. In the case of even and odd checks, the serial port will set a check bit (one bit after the data bit) to ensure that the transmitted data has even or odd logical high bits. If the data is 011, then for the even check, the check digit is 0 to ensure that the number of logically high bits is even. If it is an odd check, check bit 1, so there are 3 logical bits. The high and low positions are not really checked data, and the simple set logic is high or the logic is low. This allows the receiving device to know the status of a bit and to determine whether there is noise interfering with the communication or whether the transmission and receiving data are out of sync.
1、RS-232
RS-232 can be used in a variety of applications on industrial motherboards, such as connecting mice, printers, or modems, and can also be connected to industrial instrumentation. RS-232 is limited to point-to-point communication between the serial port of the industrial motherboard and the device, and the maximum communication distance is 15.24 meters.
2、RS-422
RS-422 uses differential signals, and RS-232 uses signals that are not balanced with ground reference. Differential transmission uses two wires to send and receive signals, which can better anti-noise and have a longer transmission interval, which has great advantages in the field use of industrial motherboards.
3、RS-485
RS-485 is an improvement over RS-422 by increasing the number of connecting devices from 10 to 32, together with the electrical characteristics at the maximum number of devices to ensure that the signal voltage is met. Superior noise immunity and multi-device capabilities, RS-485 is selected for serial connections to distributed device networks, other data collection controllers, HMIs, or other operations connected to industrial computers.
4. Differences between RS-232, RS-422 and RS-485
RS-232, RS-422, and RS-485 are serial data interface specifications that were originally developed and published by the Electronic Industries Association (EIA). RS-232 was released in 1962 under the designation EIA-232-E as an industrial specification to ensure compatibility between products from different manufacturers. RS-422 was developed from RS-232. In order to improve the shortcomings of RS-232 communication interval and low rate, the RS-422 community said a balanced communication interface, increasing the transmission rate to 10Mb/s, extending the transmission interval to 4000 feet (when the rate is less than 100kb/s), and promising to connect up to 10 receivers on a balanced bus. RS-422 is a one-way, balanced transmission specification that transmits from a single machine and accepts multiple machines, and is named the TIA/EIA-422-A specification. In order to expand the scope of use, EIA formulated the RS-485 specification on the basis of RS-422 in 1983, adding multi-point and two-way communication capabilities, that is, allowing multiple transmitters to be connected to the same bus, increasing the driving ability and resistance protection characteristics of the transmitters, and expanding the common-mode range of the bus, that is, the TIA/EIA-485-A specification.
At present, RS-232 is the most widely used serial interface for industrial motherboards. The unbalanced transmission method, the so-called single-ended communication, is designed for point-to-point (i.e., only one pair of receiving and receiving devices) communication and is suitable for communication between local devices.
RS-485, like RS-422, has a maximum transmission interval of about 1219 meters and a maximum transmission rate of 10Mb/s. RS-422 can support 10 nodes, RS-485 supports 32 nodes, RS-485 is a superset of RS-422, so all RS-422 devices can be controlled by RS-485.
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